System and method for storage system leak detection

ABSTRACT

A leak detection system and method for detecting leaks from a storage system containing a liquid, the leak detection method includes mixing one or more tracers soluble in the liquid to provide a liquid-tracer mixture within the storage system. The tracer provides a detectable component in the liquid-tracer mixture if the liquid-tracer mixture escapes the storage system through a leak. A subsurface sampling device is positioned near the storage system. The sampling device contains an adsorption material for adsorbing tracer from the liquid-tracer mixture that leaks from the system. The adsorption material is a hydrophobic, porous material. In one embodiment, a plurality of sampling devices are contained within a plurality of subsurface monitoring wells positioned around the storage system.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to leak detection systems andmethods, and more particularly, to a system and method for detecting aleak in a storage system by detecting a tracer chemical released in thestorage system.

BACKGROUND OF THE INVENTION

Underground storage tank (UST) and above-ground storage tank (AST)integrity testing has become a lucrative market, driven partly byregulatory requirements and partly by sound environmental protectionpolicy. Various systems and methods for leak testing of storage tanksand pipelines, often used for petroleum crude or refined-product storageand transport, have been introduced to meet this need. One method todetect leaks in these vessels and pipelines involves adding a specialtycompound or mixture (called a "tracer") to the product being stored ormoved that is both soluble in the product and not ordinarily present inthe product or in the environment. Subsequent detection of this tracercompound or mixture outside the vessel or pipeline system candemonstrate that the tracer mixture has escaped the system, therebyindicating the system has developed a leak.

A typical tracer release detection application involves blending atracer such as sulfur hexafluoride (SF₆) (a nontoxic, inert gas) withpetroleum-related products in a pipeline or storage tank. Halogenatednonpolar compounds, halogenated methanes, halogenated ethanes,halogenated propanes and propenes, halogenated butanes, cyclobutanes andbutenes have also been used as tracers to test fuel storage and pipelinesystems. Tracer compounds have also been used to locate the undergroundpresence and/or movement of water, soil gases, petroleum, or naturalgas. Tracers have also been used to help define the presence andcontinuity of geologic faults and permeable formations. In each case,the specialty compound or mixture, soluble in the phase or medium ofinterest and not ordinarily present in the environment, is introduced ata particular location. Successful sampling for the tracer at pointsremoved from the original release point then indicates "communication"with or "continuity" to the original release point.

Conventional tracer release detection methods to detect fuel leaksinvolve analyzing soil vapors drawn from sampling wells surrounding thefuel storage system for evidence of the tracer escaping the storagesystem. Typically, companies using tracer-related test methods locatesampling wells in the soil adjacent to fuel storage equipment. Abackground sample is usually taken prior to introducing the tracercompound into the fuel storage system to provide a baseline for the soilsurrounding the storage tank prior to the actual tracer-related test.

After installing sampling wells and taking background tracermeasurements, a technician will typically introduce one or more tracerchemicals (in either gas or liquid phase) into the fuel storage system.A predetermined mass of tracer(s) is inserted into the fuel storagesystem through a single tubing line inserted into the storage tank. Analternative tracer introduction system can involve placing an enclosedgaspermeable membrane containing a given mass of tracer(s) into thestorage tank and having the tracer release through the membrane over aperiod of time. If a storage system has a leak, the tracer may escapethe storage system with the fuel.

After some time period has elapsed, a technician uses a vacuum pumpattached either to the top of the sampling well, or to tubing placedinto the well, to draw soil vapors through the bottom end of thesampling well into a sample container. Typically, some portion of thesoil vapor sample in the container is injected into a gas chromatographequipped with an electron capture detector (ECD) to analyze the vaporsample for the presence of the tracer. These test samples are thencompared to the background samples to determine if a product leakexists.

These conventional tracer release leak detection systems have severallimitations.

For storage tank testing by tracer techniques, the desired physicalproperties of tracer compounds include high volatility. A compound'sboiling point is often a good indicator of volatility. For example, SF₆boils at about -50.7° C., making it relatively volatile at ambienttemperatures. However, the more volatile a tracer compound, the harderit is to keep that compound near a sampling point, in a samplecontainer, or in an analytical system such as a gas chromatograph, anautosampler or a gas sampling loop/injector. Volatile tracers requirecare to insure the integrity of sample containers and analytical systemcomponents Conventional tracer release detection systems avoid usingtracers more volatile than SF₆ for these reasons (and some systems usetracers even less volatile than SF₆), sacrificing analytical sensitivityand volatility for a decreased risk of accidental orcross-contamination.

Conventional tracer release detection methods using a soil vaporsampling system do not always collect adequate masses of tracer from thetracer-affected soil vapor space near or in the sampling well to detectrelatively small amounts of tracer released from a leaking storagesystem. These conventional systems only pull the soil vapor sample fromthe relatively small volume opening at the bottom end of the samplingwell. Thus, relatively small leaks may go undetected in the soil vaporsampling tracer detection systems. This problem can be exacerbated bylow-permeability soil conditions surrounding the sampling wells. Thisproblem can also limit the ability to accurately identify the locationof a leak.

Conventional tracer release detection systems must also find a way toexclude or remove water from the vapor sample when using halogenatedhydrocarbons or other electronegative compounds (such as sulfurhexafluoride) as a tracer. This must be done because an electron capturedetector has relatively high sensitivity to water. The steps taken inconventional systems to exclude or remove water from the sample canpotentially degrade the quality and/or the quantity of the sample.

Conventional soil vapor sampling tracer release detection systems usinga vacuum pump and a container to hold the extracted soil vaporstypically pull soil vapor samples from a sampling well for two to twentyminutes. The system is therefore limited to acquiring tracer sample thatis present within, or in very close proximity to, the sampling well airspace during that relatively short time period. The lack of aconcentrating mechanism in these systems can allow the tracer to migrateaway from the sampling well and be missed during the sampling process.This "short-term" sampling process can result in missing tracer releasesbecause the tracer has 1) already declined below detectableconcentrations within and near the sampling well, or 2) has migratedaway due to subsurface soil conditions, weather conditions or otherfactors.

Conventional soil vapor sampling tracer release detection methods samplesoil vapor by processes that result in a smaller mass of tracercollected. A low concentration accidental contamination of a sample hasa higher likelihood of producing a "false positive" test result with aconventional, lower-mass tracer sample than with a higher mass tracersample as can be taken using the present invention.

Conventional tracer release detection methods use a single delivery tubeto introduce tracer into the fuel. As a result, the tracer dispersesless rapidly and less uniformly throughout the fuel. This can lead tofuel leaking out that contains a less than optimal concentration of thetracer. This, in turn, can increase the difficulty of detecting thetracer, and ultimately, the leak.

Conventional tracer testing methods present a problem if a leak is atthe bottom of a tank that has collected water at the bottom of the tank.While the tracers currently used have high solubility in fuel, they havelow solubility in water, and therefore, a leak may go undetected.

SUMMARY OF THE INVENTION

The current invention improves on existing methods of tracerintroduction into storage tanks, and of sampling for tracers outsidethose tanks and their associated equipment, such as lines anddispensers.

In particular, the present invention provides a leak detection systemand method for detecting leaks from a storage system containing aliquid. The leak detection method includes mixing a tracer soluble inthe liquid to provide a liquid-tracer mixture within the storage system.The tracer provides a detectable component in the liquid-tracer mixtureif the liquid-tracer mixture escapes the storage system through a leak.A subsurface sampling device is positioned near the storage system. Thesampling device contains an adsorption material for adsorbing tracerfrom the liquid-tracer mixture that leaks from the system. Theadsorption material is a hydrophobic, porous material. In oneembodiment, a plurality of sampling devices are contained within aplurality of subsurface monitoring wells positioned around the storagesystem.

The present invention provides an important technical advantage by usingan adsorption method with a porous, hydrophobic sampling material tocollect a significantly greater mass of tracer. The adsorption materialactually attracts and concentrates the tracer within the material. Thelarger mass of tracer collected in the sample results in the ability todetect smaller concentrations of tracer and smaller storage tank leaks.

The present invention provides another technical advantage by using acollection method that allows the use of tracer compounds even morevolatile than SF₆ (such as carbon tetrafluoride (CF₄), which boils at-128.0° C.) These higher volatility tracer compounds are 1) more likelyto leave tank through a leak; and 2) more likely to migrate throughsoil. This will increase the sensitivity of the process to smallervolume leaks.

The present invention provides yet another technical advantage by usingan adsorption process that does not require any special precautions toexclude or remove water because the adsorption material will not adsorbwater. Therefore, there is no concern with water collecting in thesample to interfere with the operation of an ECD. Furthermore, thisreduces the likelihood of degrading quality and quantity of the sample.

The present invention provides another technical advantage by using along-term passive tracer collection method. The adsorption materialattracts, adsorbs and concentrates tracer to the sampling device. Thisprovides the advantage of collecting larger masses of the trace sample.Furthermore, the continuous adsorption ensures that fluctuating ordeclining tracer concentrations in the proximity of the monitoring welldo not go undetected.

The higher mass tracer sample provided by the adsorption method of thepresent invention also provides the technical advantage of reducing thelikelihood that a low level accidental tracer contamination of asampling device will lead to a "false positive" test result due to theincreased mass of the tracer sample obtained using the presentinvention.

The present invention provides still another technical advantage byintroducing the tracer into the fuel through fritted metal diffusers toprovide more rapid and uniform dispersion of tracer into the fuel. Thetracer/fuel mixture is, therefore, more even throughout the storagesystem.

The present invention provides another technical advantage by allowing amore accurate determination of a leak location due to the greater samplemass collected through passive tracer collection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a fuel storage system including one embodimentof the present invention;

FIG. 2 shows a view in elevation of one embodiment of a sampling deviceto be used in conjunction with the present invention; and

FIG. 3 shows a flow diagram of one embodiment of the sample handling andanalysis of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a typical fuel storage system 10 including storagefuel tank 12. Fuel tanks 12 may be underground storage tanks (USTs) orabove-ground storage tanks (ASTs). A typical fuel storage tank system,for example, at a retail service station or convenience store, can havetwo, three or four fuel tanks 12 (USTs) buried in a common excavation.It should be understood that the present invention relates to all typesof storage systems including pipelines and other storage systems. Itshould be further understood that the product stored in the storagesystem can include products other than fuel.

As shown in FIG. 1, USTs are usually installed in excavation 13 on topof approximately one to two feet of sand or pea gravel backfill 14.Backfill 14 can then be added and compacted around fuel tank 12 untilexcavation 13 is properly closed. USTs often have paving over thebackfilled excavation 13 with various pipes and fittings extending fromthe top of the fuel tanks through the backfill to the ground surface.Product delivery lines 16 connect product dispenser 18 to fuel storagetank 12. Product delivery lines 16 are usually placed in excavatedtrenches 15, surrounded with backfill 14, then covered to grade forpaving.

As shown in FIG. 1, tracer release detection system 20 of the presentinvention includes a plurality of monitoring wells 22, and compressedgas cylinder 26 (containing a particular tracer 46) connected to gasregulator 25, flow controller 27, and valved tee 29. Valved tee 29 canconnect to one or more transfer lines 28, each transfer line 28 coupledto a fritted metal diffuser 24 within storage tank 12.

Subsurface monitoring wells 22 can be installed when fuel tank 12 andproduct delivery line 16 are initially installed. Alternatively,monitoring wells 22 can be retro-fitted into existing tank pits and linetrenches. Backfill soil 14 around storage tank 12 and product deliveryline 16 will preferably comprise pea gravel or sand. However, some fuelstorage systems 10 use "native soil" backfill (i.e., the soil that wasexcavated from the tank pit prior to tank and line installation). Nativesoil can have low permeability that can limit tracer migration fromstorage tank 12 to monitoring wells 22. In any method of storage systemleak detection, whether internal or external in nature, the permeabilityof the backfill soil will affect the sensitivity and accuracy of leakdetection.

Monitoring wells 22 should preferably be sunk near storage system 10 andin sufficient quantity so that each monitoring well 22 is relied upon tosample from no more than a twenty foot radius around that monitoringwell 22. It should be understood, however, that a greater or lessernumber of monitoring wells 22 could be distributed around storage system10. Monitoring wells 22 should preferably be spaced around the entireperimeter of storage system 10. As shown in FIG. 1, monitoring wells 22are preferably installed at a depth equal to the depth of the bottom ofthe boundary of excavation 13 for USTs. However, the present inventionhas successfully detected tracer/fuel releases when monitoring wells 22did not extend to the bottom of excavation 13. Due to the concentratingbehavior of the sampling devices, the present invention can often detectthe fuel and tracer vapors that migrate upward through the ground.Monitoring wells 22 are preferably placed within the backfill 14,however, monitoring wells 22 can also be placed in the native soiloutside of backfill 14.

FIG. 2 shows one embodiment of monitoring well 22 having a slotted PVCcasing 30 with an opening at the surface. Monitoring wells 22 of thepresent invention can be constructed to have a casing with a cylindricalcross-section of nominal diameter of one to four inches. Monitoringwells 22 can include PVC casings 30 that have slots cut in them toreceive tracer and fuel vapor and liquid. The slots in the PVC casing 30of monitoring well 22 can be horizontal slots approximately 0.010 to0.020 inches wide and approximately 0.5 to 1.0 inches long. The slotscan be cut around the diameter of the monitoring well 22 and can bespaced vertically along the length of the casing approximately 0.20 to0.50 inches apart from one another. The slots in monitoring wells 22allow tracer/fuel vapor to more easily migrate into the monitoring wells22 to contact the sampling devices 40. Each monitoring well 22 includessampling device 40 suspended within the monitoring well to collecttracer 46. An example of a monitoring well that can be used with thepresent invention is disclosed in U.S. Pat. No. 5,003,813, which isincorporated herein by reference.

As shown in FIG. 2, subsurface sampling device 40 is suspended in eachmonitoring well 22 through the surface opening of the monitoring well22. The sampling device 40 is preferably suspended at a depth ofapproximately two to four feet below grade. In an alternativeembodiment, the sampling devices 40 can be suspended in a subsurfaceexcavation near the storage system 10 (not contained within monitoringwells 22). In yet another embodiment, the sampling devices 40 can besuspended above ground, rather than subsurface. Sampling devices 40preferably comprise stainless mesh cylindrical containers, eachcontainer holding a mass (for example, 0.1 to 2.5 grams) of porous,hydrophobic adsorption material 44. An example of a porous, hydrophobicmaterial 44 is an activated carbon. A preferred porous, hydrophobicadsorption material 44 is a synthetic polymer such as that disclosed inU.S. Pat. No. 4,863,494 which is incorporated herein by reference. Thisporous, hydrophobic material 44 reversibly adsorbs and concentrates bothfuel and tracer vapors that contact the porous hydrophobic material 44over the period of exposure.

Fuel storage system 10 can continue in operation during the entiretracer release detection process of the present invention includingduring a retro-fit installation of monitoring wells 22. The tracerrelease detection process of the present invention requires analyzingthe test site (i.e., the area around storage system 10 where monitoringwells 22 are located) for the presence of any tracer compounds andpossibly fuel prior to purposeful tracer introduction into fuel tank 12.This background test serves to establish a baseline of how much traceris present at the test site prior to fuel tank 12 leak testing. Also,testing for the presence of particular fuel types can provide valuableinformation about the site contamination history. These backgroundsamples will provide baseline samples to compare to test samples exposedto the tracer and fuel during tracer release and leak detection testing.The background testing can be performed using the method described bythe present invention, or alternatively, by conventional methodsincluding soil vapor samples such as those described in the backgroundof the invention.

After performing the background testing, an unexposed sampling device 40is suspended in each monitoring well 22. Each monitoring well 22 is thensealed. Tracer chemicals 46 are chosen for introduction to each fueltank 12 on site. Examples of tracer chemicals include, but are notlimited to, sulfur hexafluoride, halogenated hydrocarbons, and krypton.Tracers 46 useful in the application of the present invention includethose with high solubility, relatively low boiling points, andrelatively high vapor pressures at ambient temperatures. Tracers 46 arepreferably non-polar compounds, so that they are well-suited toadsorption by, and concentration on the hydrophobic, porous adsorptionmaterial. Tracers 46 include both gaseous and liquid compounds. For asite with multiple storage systems 10 or multiple storage tanks 12, adifferent tracer 46 can be introduced into each storage tank 12 in orderto differentiate between tanks if tracer 46 release is detected.Additionally, two or more tracers 46 may be added to a single tankwhere, for example, the first tracer 46 is soluble in the liquid storedin the tank and the second tracer 46 is soluble in water.

To introduce tracer 46 to the fuel, a tracer introduction systemincluding a container 26 containing tracer 46 can force tracer 46 toflow through the gas regulator and flow controller, into transfer lines28. The tracer 46 can be introduced into the liquid in the storagesystem 10 in either a gas or a liquid state. Tracer 46 exits transferline 28 through fritted metal diffuser 24, which has been placed nearthe bottom of tank 12 The present invention uses fritted metal diffusers24 placed near the bottom of storage tank 12 to uniformly dispersetracer 46 to allow rapid mixing of tracer 46 with the liquid (forexample a hydrocarbon fuel) throughout the tank 12 to produce aliquid-tracer mixture within the storage system 10. Fritted metaldiffusers 24 provide rapid mixing and more uniform dispersion because asthe tracer gas or liquid passes into the metal diffuser 24, it breaksthe tracer into many small gas bubbles or liquid droplets. This allowstracer 46 to exit the fritted metal as many tiny droplets or tiny gasbubbles, depending on the phase. In this way, tracer 46 dissolves intothe liquid stored in the tank much more rapidly. This process allows theliquid-tracer mixture to more rapidly reach an average concentration oftracer 46 throughout the liquid. In contrast, if gas-phase tracer 46 isintroduced through 1/8 inch or 1/16 inch tubing without fritted exitdiffusers, large bubbles issue from the tube end and rise to the liquidsurface in a tank. Much of the tracer would remain in vapor phase in thetank, to be lost to atmosphere through vents required as part of thetank system.

Tracer 46 moves through transfer lines 28 into tank 12 at flow ratesranging from less than 30 milliliters to over 400 milliliters perminute. The total mass of tracer 46 needed to test a particular fuelstorage system 10 depends on a variety of factors, including but notlimited to, total tank volume, tank volume typically in use, approximatevolume of fuel moved through tank in a month-long period, volatility ofthe fuel product, volatility of the tracer compound, permeability ofbackfill or native soil, ground-water level, soil moisture, and fractionof organic carbon in backfill or native soil. Typical masses of tracer46 can range from less than 25 grams to greater than 1,000 grams perfuel storage tank 12.

Once the proper mass of tracer 46 has been introduced to storage system10, the container 26 is removed from transfer lines 28. A nitrogencompressed gas cylinder (not shown) can then be connected to transferlines 28. Nitrogen can be flowed through transfer lines 28 and frittedmetal diffusers 24 for a period of time (typically several minutes), toforce approximately all tracer 46 out of transfer lines 28 into tank 12.Transfer lines 28 and metal diffusers 24 are then removed from tank 12,and tank fill pipe 32 is sealed.

Sampling device 40 will remain exposed for a period of time. Porous,hydrophobic adsorption material 44 in sampling devices 40 willcontinuously attract and adsorb tracer 46 that leaks from tank 12. Soilvapors, occupying the spaces or voids between soil particles, arepredominantly made up of the permanent gases (nitrogen, oxygen, argonand carbon dioxide) and water vapor. None of these compounds is adsorbedto any great degree by the porous, hydrophobic adsorption material 44. Anon-polar vapor-phase molecule, such as a tracer 46, in the soil vaporexerts a vapor pressure and partial pressure. If such a tracer moleculecomes into contact with the adsorption material 44, it physically bondsto the surface of the adsorption material 44, changing phase from vaporstate to "sorbed" state. As part of this phase change, the tracermolecule no longer exerts a vapor pressure or partial pressure in thesoil vapor, resulting in a lower concentration of that type of moleculenear the sampling device 40. More molecules of that type then move intothe affected volume, to re-equilibrate partial pressure andconcentration. An equilibrium is typically reached after the adsorptiondevice has concentrated the available tracer to levels hundreds orthousands of times greater than the ambient tracer concentration.Typical tracer sampling time may be as short as one to three days orlonger than two to three weeks after tracer 46 introduction into thestorage system 10. Variables that affect this sample exposure timeinclude those previously identified as affecting the mass of tracer 46introduced into a tank 12.

The present invention allows increased tracer mass collection using amonitoring well 22 having a slotted PVC casing. Whereas conventionalsystems often use sampling wells with a solid-wall casing and pull avapor sample only from the bottom opening of the sampling well, thepresent invention samples using a monitoring well 22 having a slottedcasing The slotted casing monitoring well 22 exposes a sampling device40 to soil vapors from the entire vertical extent of the monitoring well22. This is especially important in view of the fact that most tracercompounds are denser than air. The majority of a tracer's mass will,therefore, sink through the soil vapor space, tending to concentrateagainst soils or rock of lower permeability, or ground water, typicallydeeper than the region in which sampling is taking place.

FIG. 3 shows a flow diagram of the sample device handling and analysis.As shown in step 91, once an appropriate amount of time has elapsed,exposed sampling device 40 containing adsorption material 44 is removedfrom each monitoring well 22. In step 92, the sampling device 40containing adsorption material 44 is sealed in a container, for examplein a glass vial having a screw-top or crimp-top lid. In step 93, allpotential leak paths on the vial are sealed using a wrap. Potential leakpaths include, for example, the screw top lid seals with the glassthreads, seal in the lid presses, seal against the top of the glassvial, or the seal against the open top in the lid. A portion of the soilvapors in the sampling device 40, including a portion of the tracermass, will separate from porous hydrophobic material 44 into the vaporin the container enclosing sample device 40. The wrap used to coverthese potential leak areas can include a paraffin wax wrap, a metalwrap, or both. The sealed sampling device 40 is then transported to thetesting facility in step 94. This testing facility can include a remoteor on-site testing facility. A nonexposed sampling device 40 can beplaced in each monitoring well 22 at this time to allow confirmationsampling at the same monitoring well 22, if necessary.

In step 95, a sample is then obtained from the container enclosingsampling device 40. The sample may be obtained from within the containerin one of at least two ways: 1) the sample can comprise the headspacevapor in the container enclosing sampling device 40, or 2) the samplecan comprise a liquid organic solvent aliquot obtained after the solventis injected into the container to desorb the tracer from the hydrophobicmaterial into the solvent. For a headspace vapor sample, a headspaceauto-sampler 60 actually samples this headspace vapor for tracer.Without the paraffin (or other) wrap as a secondary hermetic seal, SF₆(or other tracers 46 used) will escape the container and pose a threatfor accidental contamination of other samples or system components.Using the paraffin or metal wrap method, tracer compounds even morevolatile than SF₆ can be used and adsorbed, such as carbon tetrafluoride(CF₄), which boils at -128.0° C. The present invention can use tracercompounds with boiling points lower than -128 degrees Celsius (forexample, krypton) and greater than 150degrees Celsius (for example,three-hexanol alcohol).

In step 96, the sample is analyzed. For a headspace vapor sample, thevapor sample, still sealed in a 40-milliliter (ml) vial, for example, isloaded into a headspace auto-sampler connected to a gas chromatograph. Aheadspace auto-sampler is the preferred device for analyzing a headspacevapor sample. By a set of automated steps, sample of headspace vaporsfrom the container enclosing sampling device 40 is injected into gaschromatograph and separated on an analytical column in thechromatograph. The tracer 46 is then detected as discrete compoundpeak(s) at a detector. The detector can be an electron capture detector,which is the most sensitive detector currently available for detectingsulfur hexafluoride and the halogenated hydrocarbons (excluding thefluorinated hydrocarbons). For fluorinated hydrocarbons such as carbontetra fluoride, a negative-ion mass spectrometer is currently the mostsensitive detector. The detector can also include other massspectroscopic detectors, a flame ionization detector, a heliumionization detector, a thermal conductivity detector, an electrolyticconductivity detector, a chemiluminescent detector or otherelement-specific detectors (oxygen, nitrogen, or sulfur, for example).

In the alternative method of washing the sampling device 40 with anorganic solvent to form a liquid aliquot sample, a measured volume ofliquid organic solvent is injected into the sealed vial holding thesampling device 40. The sampling device 40 and organic solvent areagitated for a period of time, to more completely desorb tracer 46 fromthe adsorption material 44, within the sampling device 40 to form liquidsample. After agitation, the liquid sample, now containing tracer 46from the sampling device 40, is removed from the sealed vial and placedinto an autosampler vial. The autosampler vial is then placed into anautosampler, for subsequent injection by a set of automated steps into agas chromatograph for analysis.

Conventional sampling systems, using a vacuum pump to collect soilvapors into a sample container, must attempt to eliminate water vapor orliquid from the sample, as water interferes greatly in the operation ofan ECD. These systems often include the use of water-permeablemembranes, to preferentially remove water from an air stream. Incontrast, the present invention uses the hydrophobic (water-rejecting),porous adsorption material 44 (such as a synthetic polymer) to passivelyadsorb and concentrate the tracer compounds. Because the material ishydrophobic, no water will collect in absorbent material 44 and onlysmall amounts of water will absorb onto the surface of the stainlessmesh exterior of sampling device 40. Therefore, no additional steps toexclude water from the sample, or from the analytical process, need betaken.

In step 97, the data obtained in step 96 is analyzed to determine if aleak has occurred. Numerical and spatial comparison of tracerconcentration data obtained from the analysis of potentiallytracer-exposed sample 50 (headspace vapor or liquid aliquot sample) withthe concentrations of tracer present in the background samples 70demonstrates whether tracer 46 has appeared outside the tank 12 orproduct line 16 in amounts greater than previously detected. A higherconcentration of tracer 46 in the exposed sample (as compared tobackground sample 70) can indicate a tank 12, product line 16, or even adispenser has leaked fuel into the ground near a monitoring well 22. Theresults of this analysis can then be reported to the client.

By comparing concentrations of tracer 46 at different monitoring wells22, the approximate location of a leak may be estimated. For example,simple triangulation in soils of similar permeability points to a leaklocation in two dimensions. This can be improved upon based on siteconditions, tank and other equipment geometries, and other factors. Thenature of the adsorption material 44 and the extended sampling period ofthe present invention concentrates a more representative amount oftracer 46 at each monitoring well. Thus, a monitoring well closer to aleak will concentrate a larger mass of tracer, providing more accurateleak location information This more accurate leak location informationcan eliminate the need to excavate an entire tank pit or line trench torepair a leak. Rather, only a portion of the pit or trench may requireexcavation in order to repair a damaged storage system 10.

A storage system 10 can also include multiple storage tanks, any numberof which may have leaks. One solution to this problem would be tointroduce a different tracer compound into each tank. The presentinvention will adsorb a variety of tracer compounds. Thus, the presentinvention will allow a user to determine which tanks in a multiple tankstorage system have leaks.

Furthermore, conventional tracer testing methods present a problem if aleak is at the bottom of a tank that has collected water at the bottomof the tank. In a fuel storage tank that has collected water, the waterwill separate to form a water level below the fuel level due to thedensity of water as compared to fuel. While the tracers currently usedhave high solubility in fuel, they have low solubility in water. Thus,the tracer will not dissolve in the water level at the bottom of thetank. If a leak develops at the bottom of the tank, the water will firstescape, but no tracer will escape with the water. Thus, the leak will goundetected until the water level decreases to the point that fuel beginsto leak. By using a tracer that is soluble in water and is adsorbable bythe adsorption material (such as methylene chloride), the presentinvention can detect the leak.

Another important advantage of the present invention is the increasedability to detect relatively small leaks. By exposing sampling device 40for an extended period of time, the process of the present inventionrelies on the porous, hydrophobic material 44 to selectively partitionto its surface area the available tracer compounds. This sampling methodof the present invention has several advantages over conventionalshort-term method of drawing of soil vapors into a sample containerusing a vacuum pump. The present invention allows the use of highervolatility tracer compounds such as carbon tetrafluoride. Conventionalsystems avoid using these higher volatility compounds, having lowerboiling points, due to the fact that the more volatile compounds aremore difficult to keep near a sampling point (such as in the soil vaporsof a conventional sampling well). Because the present invention uses asampling method that collects (through adsorption) the tracer on anadsorption material 44, the fact that the volatile compound will moreeasily migrate does not affect the present invention's ability tocollect a tracer sample as dramatically as it does conventional tracerdetection methods. Using a more volatile tracer provides the advantagethat the tracer will more rapidly pass out of a leak in the storagesystem and more rapidly migrate to the sampling device in the monitoringwell.

Furthermore, the present invention will likely collect a greater amountof tracer from a leak. Conventional tracer detection methods usingshort-term sampling by vacuum pump obtain sample sizes ranging from 1 to50 liters of soil vapor. The short-term sampling runs the risk ofmissing a tracer's presence near a sampling well, since that tracerconcentration in soil vapor may vary with changing soil humidity,rainfall events, shallow soil temperature changes, changes inatmospheric pressure, changing level of liquid product in the storagetank of interest, and other factors. In contrast, sampling over a periodof many hours or days, by passive adsorption, helps to "average out"these variable factors, as well as obtain a larger mass of sample foranalysis. Tracer masses equivalent to several hundred liters of soilvapor are routinely obtained using the passive adsorption on a poroushydrophobic material 44 of the present invention.

To illustrate the sensitivity of this tracer release leak detectionprocess of the present invention, an electron capture detector candetect sulfur hexafluoride (one type of tracer 46) at a mass as low as 1picogram (1×10⁻¹² gram) in a sample injected into a gas chromatograph.If 250 grams of sulfur hexafluoride are mixed into 8,000 gallons ofdiesel fuel in a tank, each gallon of fuel may contain approximately3.125 million picograms of sulfur hexafluoride. If, over a three-dayperiod, 0.1 gallon of diesel fuel escapes from a break in the tank, aproduct line, or a dispenser, and moves into the backfill, there can beas many as 312,500 picograms of sulfur hexafluoride available in thebackfill. Because the porous, hydrophobic material used in samplingdevices tends to adsorb and concentrate sulfur hexafluoride, adetectable portion of the escaped sulfur hexafluoride will likelycollect on one or more sampling devices.

To further illustrate, suppose that one-hundredth of the sulfurhexafluoride collects on an individual sampling device. This means thatabout 3,125 picograms of sulfur hexafluoride are available on thesampling device. Once the sampling devices are retrieved from the fieldand sealed in individual 40-ml vials, some of the sulfur hexafluoridedesorbs from the porous, hydrophobic sorbent and becomes a part of thevapor in the vial. For the sake of numerical argument, suppose that onetenth of the sulfur hexafluoride from the sampling device desorbs intothe vial vapor space, or headspace. This may represent as much as 312picograms of sulfur hexafluoride in the headspace The gas chromatographauto-sampler pulls what may be as much as one milliliter of vapor fromthe vial's 40-ml total volume, and injects that vapor into the gaschromatograph. This results in seven to eight picograms of sulfurhexafluoride in the sample, to be detected at the gas chromatograph'sdetector. Conventional tracer release detection devices may or may notbe capable of detector this small a volume of tracer, from this small aleak.

An actual performance study using the present invention was conductedwith the New Mexico Environment Department's Underground Storage TankBureau. At a site in Gallup, NM, eight monitoring wells were placed intoundisturbed native soil. These wells were positioned as follows: onewell, in the center of a circular pattern (the "central well") wasplaced to a depth of about 14 feet below ground surface (BGS), and wasscreened from 13 to 14 feet with slots in the casing. Casing withoutslots extended from 13 feet BGS to approximately one foot above groundsurface. Around the casing, sand was packed from 14 feet BGS back toabout 11.5 feet. Above the sand, bentonite clay was packed back to 0.5feet BGS, to assure that no high permeability path for vapor travelexisted above 11.5 feet BGS along the casing or the boring edges. Thewell was then finished to grade with concrete.

Four monitoring wells were placed in a circular pattern, spaced 90degrees apart from one another at distances of six feet from the centralwell. Each of these wells was sunk to 9.5 feet BGS and screened fromabout 9.5 feet to 4.5 feet BGS. Around each well casing, sand was packedfrom 9.5 feet to about 4 feet BGS. Above the sand, bentonite clay waspacked back to about 0.5 feet BGS. The well was finished to grade withconcrete.

Four more sampling wells were placed nine feet from the central well, ina similar configuration to those placed at six feet from central well.

At a distance of 15 feet from the central well, one monitoring well wasplaced, identical in construction to those placed at six and nine feetfrom the central well. This well lay northwest of the central well, on acommon axis with four sampling wells and the central well. The remainingfour sampling wells lay on a northeast-southwest axis, passing throughthe central well.

Upon completion of all wells, sampling devices were placed in each well.After a two-week exposure period, all "background" sampling devices wereremoved and tested for presence of sulfur hexafluoride. No sulfurhexafluoride was detected at greater than five picograms per liter ofvapor.

Clean sampling devices were placed in all sampling wells, with twodevices placed in each of the sampling wells at six and nine feet fromthe central well. These paired devices were set at depths of three andnine feet BGS.

After all sampling wells were properly sealed, sulfur hexafluoride wasflowed into the central well, or sample introduction well. It was flowedat 200 milliliters per minute for 20 minutes, which put approximately 25grams of sulfur hexafluoride in the sample introduction well.

The monitoring wells were placed to the depths chosen because a lowpermeability soil, with high clay content, was present at depths ofabout 9.5 to 12 feet below ground surface. This soil was sampled andanalyzed for hydraulic conductivity and other parameters. One USEPA-approved method of UST monthly monitoring requires soil to have ahydraulic conductivity greater than 0.01 centimeter per second (cm/s).This tight soil was found, through independent testing, to have ahydraulic conductivity of 2.6×10⁻⁸ cm/s. This indicated that someportion of the soil column between 9.5 and 12 feet BGS was approximately300,000 times less permeable than the 0.01 cm/sec "minimum" hydraulicconductivity called for by EPA regulations in ground-water surfacemonitoring. In these low-permeability soil conditions, where the tracercompound had to travel out of the central well casing, upward at leasttwo feet through this very low permeability soil, then laterally to themonitoring wells, sampling devices were analyzed after 24 hours, sevendays, 14 days and 28 days. After only 24 hours, the tracer was detectedon all sampling devices at all wells except for the one 15 feet away,which was not sampled until two weeks later. At seven days, tracer wasdetected at all wells six and nine feet from the central well. At 14days and 28 days, the tracer was detected on all sampling devices at allwells, at concentrations as high as 25 micrograms per liter (25 millionpicograms per liter) of headspace.

In UST backfills, permeability of disturbed native soil--which is theworst soil condition encountered--is believed to be much greater (morefavorable for vapor movement) than 10⁻⁸ cm/s. For the example of sulfurhexafluoride mixed in diesel fuel described above, where only 3.125milligrams of tracer would get outside a tank if 0.1 gallon of diesel isreleased, the mass of sulfur hexafluoride is one ten-thousandth thatreleased in the New Mexico study. Yet the permeability of typicalbackfills is so much greater than seen in the New Mexico study that eventhis small mass of sulfur hexafluoride will likely be detected throughuse of the present invention.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade hereto without departing from the spirit and scope of the inventionas described by the appended claims.

What is claimed is:
 1. A leak detection system for detecting leaks froma storage system containing a liquid less dense than water in whichwater has collected at the bottom of the storage system to form a waterlevel and a separate liquid level, comprising:a tracer, the tracercomprising a substance soluble in the liquid to provide a liquid-tracermixture upon introduction into the liquid, the tracer operable toprovide a detectable component in the liquid-tracer mixture; a seconddetectable tracer, soluble in water, operable to mix with water andescape with the water through a leak in the storage system within thewater level; a tracer introduction mechanism for introducing the tracersinto the liquid; and a sampling device positioned near the storagesystem, the sampling device containing adsorption material for adsorbingtracer from the liquid-tracer mixture and the water-tracer mixture. 2.The system of claim 1, wherein the first tracer and the second tracercomprise a relatively volatile compounds capable of adsorption by thesynthetic polymer adsorption material.
 3. The system of claim 1 furthercomprising:a plurality of subsurface monitoring wells positioned nearthe storage system, each monitoring well containing a sampling device,wherein each monitoring well further comprises; a casing having agenerally cylindrical cross section; a surface outlet through which thesampling device can enter and exit the monitoring well; a plurality ofslots formed along the vertical extent of the casing to allow tracer tomore easily migrate into the monitoring well to contact sampling device;and wherein the plurality of subsurface monitoring wells are distributedabout the storage system such that each monitoring well monitors lessthan a twenty foot radius about that monitoring well and furtherdistributed in relatively close proximity to the storage system, andfurther wherein the plurality of monitoring wells are sunk in thebackfill to a depth equal to the depth of the excavation; and whereinthe sampling device further comprises a removable stainless steel meshcylindrical container suspended within monitoring well below grade forholding the adsorption material.
 4. The system of claim 1, wherein theliquid is a hydrocarbon fuel.
 5. The system of claim 1, wherein thetracer comprises a compound selected from the group consisting ofhalogenated compounds, sulfur hexafluoride, chlorotrifluoromethane, andcarbon tetrafluoride.
 6. The system of claim 1, wherein the tracercomprises relatively volatile compounds having boiling points in therange of approximately -73 degrees Celsius to -128 decrees Celsius andgreater than 150 degrees Celsius.
 7. The system of claim 1, wherein thetracer introduction system further comprises;a tracer container operableto hold the tracer prior to introduction into the storage system: aplurality of transfer lines, each transfer line coupled at a first endto the container, each transfer line operable to provide a path for thetracer to flow from the container to the storage system, the transferlines having a second end placed within the liquid near the bottom ofthe storage system; a plurality of metal diffusers, each metal diffusercoupled to the second end of a transfer line to diffuse the tracer,thereby providing more uniform and rapid mixing of the tracer with theliquid; and a purge container operable to hold an inert purge gas, thepurge container operable to connect to the plurality of transfer lines,the purge gas operable to flow through the transfer lines after thetracer has been introduced to force approximately all of the tracer outof the transfer lines and into the storage system.
 8. The system ofclaim 1, wherein the adsorption material comprises a synthetic,hydrophobic, porous polymer material containing at least 50% divinylbenzene.
 9. The system of claim 1, further comprising an analysis systemfor analyzing the adsorption material, the analysis system comprising adetector to detect the presence of tracer in the adsorption material.10. The system of claim 9, wherein the detector comprises an electroncapture detector, and wherein the electron capture detector can detectthe tracer without having to first perform a water purge operation. 11.A method for detecting a leak from a storage system containing a liquidless dense than water in which water has collected at the bottom of thestorage system to form a water level and a separate liquid level, themethod comprising;mixing a first tracer with the liquid in the storagesystem to form a liquid-tracer mixture; mixing a second tracer, thesecond tracer being soluble in water, with the water to form awater-tracer mixture at the bottom of the storage system; placing asampling device containing an adsorption material near the storagesystem; adsorbing the first tracer vapor onto the adsorption materialwithin the sampling device; adsorbing the second tracer vapor from thewater-tracer mixture on the adsorption material; and analyzing theadsorption material for the presence of the second tracer to determineif a leak is present in the storage system.
 12. The method of claim 11wherein sealing the sampling device within a container further comprisessealing the container with a wrap, the wrap operable to form a hermeticseal to contain the vapors from the adsorption material within thecontainer.
 13. The method of claim 11 wherein obtaining a sample furthercomprises obtaining a sample of headspace vapor from the adsorptionmaterial contained within the sealed container.
 14. The method of claim13 wherein analyzing the sample further comprises analyzing theheadspace vapor contained within sealed container, the analyzing furthercomprising;loading the sealed container into a headspace autosamplercoupled to a gas chromatograph; injecting headspace vapors within thesealed container into gas chromatograph; separating the headspace vaporson an analytical column in the chromatograph; and detecting tracercontained within the headspace vapors with a detector.
 15. The method ofclaim 14 wherein detecting tracer in the headspace vapors furthercomprises using an electron capture detector to detect tracer containedwithin the headspace vapors.
 16. The method of claim 11 whereinobtaining a sample further comprises washing the adsorption materialwith an organic solvent to form a liquid aliquot sample.
 17. The methodof claim 11, further comprising;placing a plurality of subsurfacemonitoring wells around the storage system in relatively close proximityto the storage system and distributing the plurality of monitoring wellsabout the storage system such that each monitoring well monitors lessthan a twenty foot radius about that monitoring well; suspending asampling device within each of the plurality of monitoring wells atbetween two and four feet below grade; and forming the plurality ofmonitoring wells to have a casing with a generally cylindrical crosssection, a surface outlet through which the sampling device can enterand exit the monitoring well, and a plurality of slots along thevertical extent of the casing to allow tracer vapor from theliquid-tracer mixture to more easily migrate into the monitoring well tocontact adsorption material within the sampling device.
 18. The methodof claim 11, further comprising purging the transfer lines with an inertgas by flowing the inert gas through the transfer lines after the tracerhas been introduced to force approximately all of the tracer out of thetransfer lines and into the storage system.
 19. The method of claim 11,wherein mixing a soluble tracer with the liquid further comprises mixinga tracer compound having a boiling point in a range of approximately -73degrees Celsius to -128 degrees Celsius with a hydrocarbon fuel in ahydrocarbon fuel storage system.
 20. The method of claim 11, whereinmixing a soluble tracer with the liquid further comprises mixing atracer compound having boiling points greater than 150 degrees Celsiuswith a hydrocarbon fuel in a hydrocarbon fuel storage system.
 21. Themethod of claim 11, wherein placing a sampling device containing anadsorption material further comprises placing a hydrophobic, porous,synthetic polymer material containing at least 50% divinyl benzenewithin the sampling device.
 22. The method of claim 11, wherein theanalyzing further comprises analyzing the adsorption material for thepresence of the first tracer to determine if a leak is present in thestorage system.